Papermaking processes strongly require an improvement in the retention of fines and fillers in pulp slurry in the process in order to increase their productivity. The retention in this context means the proportion of the amount of the solids, such as fines and fillers, which do not pass through the pulp layer formed on the wire in papermaking, to the entire amount of the solids. As the proportion becomes greater, the retention becomes better.
On the other hand, the closed-system treatment of white water, in which white water is circulated and reused in the preparation of pulp slurry to decrease the amount of water discharged to the outside of the system, has been forwarded. In this closed system, since white water once used in the papermaking is treated and reused, it becomes important to reduce the amount of fines and fillers which are in the white water and do not remain on the paper, as much as possible.
In order to make the fines and fillers remain on the paper have been proposed various methods of adding to pulp slurry retention aids which comprise a silica sol, and an additive such as a cationic or anionic high molecular weight poly-acrylamide compound. Examples of the methods are as follows:
(1) using a cationic starch and a colloidal silicic acid in pulp slurry that includes a filler, from which slurry paper is made (in JP57-51900, A)
(2) using a cationic or amphoteric carbohydrate and a colloidal silicic acid modified with aluminum (in JP2-19238, A)
(3) using a cationic poly(meth)acrylamide compound and a colloidal silicic acid (in JP62-15391, A)
(4) using a retention aid comprising a cationic polyacrylamide, and aluminum silicate or a colloidal silicic acid modified with aluminum (in Japanese Translated PCT Application Publication No. 63-500190)
(5) using a retention aid such as that including a cationic polyacrylamide compound or a polyethylene imine, and a polyaluminum compound together with a colloidal silicic acid and a colloidal silicic acid modified with aluminum (in Japanese Translated PCT Application Publication No. 1-502519)
(6) using a cationic synthesized compound including a cationic polyacrylamide retention aid, an aluminate, and an inorganic colloid (in JP2-99692, A)
(7) using an aluminum compound, a cationic polymer retention aid, and a polymeric silicic acid having a specific surface area of 1050 m2/g or more (in JP6-11957, B)
(8) using a water-soluble cationic polymer and a water-soluble polysilicate microgel (in JP2-229297, A)
Although the employment of the aforementioned methods which use a composition including silica sol and the additive can enhance the retention to some extent, the enhancement was not sufficient and there was room for further improvement.